Screws of this type are well-known and are generally referred to as "angle-drive" screws. As is known, these screws have substantial advantages, arising from the fact that they can be used in situations where ordinary screws would be inaccessible with a conventional screwdriver, such as in a surface very close to an internal corner.
The known types of angle-drive screws are generally of the cross-head type, the screwdriver recess being shaped to allow the tip of a cross-head screwdriver such as those known by the trade name "Phillips" or "Pozidriv" to enter the recess at a relatively large angle, of the order of 20.degree. or 30.degree.. To prevent the tip of the screwdriver from lifting out of the recess whilst the screw is being driven, the walls of the recess are formed with undercuts.
In the prior art several different methods are known for forming angle-drive screws with these desired characteristics.
A first method is described in GB 2129348 which discloses a two-stage process for making angle-drive screws. The first step comprises the formation of a die-cast or moulded screw blank having a domed head formed with a generally cross-shaped screwdriver recess in its end surface. The screw blank is subsequently subjected to an axially-directed impact or squeezing force applied to its domed end, which acts to flatten the end surface of the screw head. As the upstanding segments between the arm portions of the cross-shaped recess are compressed into the recess they form overhangs resulting in undercuts in the recess walls.
FIG. 1a shows a section through an example of a screw blank of this type, and FIG. 1b illustrates the resulting finished screw after the compression force has been applied. As can be seen from these Figures the bottom surface of the recess curves upwardly from the axial centre towards the circumference of the screw head. The resulting cupped shape provides extra depth for insertion of the tip of a screwdriver when driving the screw, which is advantageous.
Although providing a screw which functions satisfactorily the two-stage process described is an expensive manufacturing technique because of the secondary processing required to apply the axial compression force. Screws produced by this method are therefore relatively costly.
European Patent 0112030 describes an alternative method for manufacturing angle-drive screws which does not require the secondary action described above. This document discloses a one-stage moulding or casting process in which two main mould-parts define the screw cavity and are arranged to be separable along a plane including the screw axis. In known fashion the screw is removed by separation of the mould parts. One of the mould-parts is provided with a blade portion which extends in a direction perpendicular to the plane of separation of the mould-parts to form an enlarged recess in the head of the finished screw, the recess extending transversely of the head and opening in a slot at its periphery. At the same time a further mould-part is arranged to be retractable in an axial direction from the head of the screw. This mould part is arranged to form the cross-shaped opening in the end surface of the head, this recess connecting with the enlarged transverse recess below. The finished screw is consequently formed with a cross-shaped opening in the end surface of the screw head and an enlarged recess underneath it so that the walls of the recess are effectively undercut.
Angle-drive screws produced by this method are cheaper than those made by the previously described two-stage process. However, they do have some disadvantages associated with the method of manufacture and the use of the blade-shaped mould-portion to form the enlarged recess in the head. Firstly, the resulting slot formed in the side of the head is relatively large and gives rise to weak points which may lead to fractures in the screw head during use. Also, because the blade must be withdrawn through the slot at the end of the moulding process it cannot be used to form a curved bottom surface in the recess, as described in relation to the screw formed by the two-stage process outlined above. The recess therefore does not have the extra depth for insertion of the screwdriver tip provided by the cupped shape.
An example of the second type of prior art angle-drive screw is shown in FIGS. 2a and 2b. FIG. 2a is a plan view of the end of the screw head and FIG. 2b is a sectional view along the line B--B' shown in FIG. 2a.